What to do about order()

[tshort]

order() is no longer a Base function. It has been superseded by sortperm. Also, we don't have something like R's order that can order by multiple vectors. We use order in a few places, so how should we fix this? Options might include:

1. Replace order with sortperm.
2. Define order based on sortperm. If so, how much of sortperm's flexibility should we pull in?
3. Extend either of sortperm or order to handle multiple vector ordering.
4. Extend sortby to cover multiple columns.

I prefer having order (I don't like the name sortperm).

[johnmyleswhite]

I really dislike the name sortperm now that it's become the name for order and not something more interesting. I would prefer that we extend sortby because I feel like order should be kept as an atomic function that doesn't do any ordering, but tells you the operations needed to put things in order. In contrast, I think of sortby as doing the ordering.

[tshort]

I think we're on the same page. Okay to have order and sortby with sortby calling order?

[johnmyleswhite]

Definitely. I would like order to be restored as the primitive.

[StefanKarpinski]

Order strikes me as an extremely overloaded term. I have to confess that I don't really get the objection to sortperm. Is the issue its new behavior or the name itself?

[johnmyleswhite]

My concern is twofold:

• The behavior is strange, because we now use a Matlab jargon term but don't perform the equivalent Matlab action.
• The name is strange because sortperm isn't actual a mathematical term: it's Matlab jargon that R users would never even consider trying out when looking for the order function. order is a proper mathematical term, although, as you've said it's badly overloaded. But it is the term people would use in a textbook, unless they used the term rank, which is really the term used in most statistics textbooks. That term is also overloaded, but it is consistent with the principle of Calling Things What They Are Called.

[StefanKarpinski]

I don't think that sortperm is actually from Matlab (@ViralBShah, @JeffBezanson?), I think we just chose the name at some point and it stuck – previously for sorting and returning a permutation. In Matlab you just use the sort function with two arguments for that and do [~,p] = sort(v) for what sortperm currently does. I like the name because it gives a permutation of v that puts it in sorted order, which is, to me, pretty intuitive.

[ViralBShah]

I think order has too many different meanings for different branches of mathematics. sortperm is not actually a matlab name and is something I started using when I first wrote sort.

Perhaps the DataFrames package can have an order function, which also takes care of the NAs and such.

[StefanKarpinski]

We should use the same name in base and data frames.

[tshort]

When I first saw sortperm, I thought "sort permanently". I only made the connection to order by digging through git history. Even when I discovered perm meant permutation, I had to go to the dictionary to find what it meant. Not a big deal, though. With the new sortby capability that Kevin Squire just added, that reduces the need for order.

[tshort]

I'm going to close this issue. Kevin's additions (#177) get us consistent with base and get us multi-column sorting.